It has been widely recognized that exhaust emissions from internal combustion engines are directly hazardous to the health of living organisms and to the environment. As a result, diesel engine emissions of nitrogen oxides (NO.sub.x) and particulates from diesel engines, which traditionally have been afforded more lenient treatment than Otto cycle engines, are increasingly being regulated by governmental agencies.
Accordingly, various techniques are under active consideration to reduce noxious emissions emanating from these engines. Concepts include engine modifications and improvements, alternate and reformulated fuels (i.e., hydrogen), and exhaust after treatment devices such as traps, catalyzed traps and catalytic converters.
Unfortunately, engines modified or improved to reduce NO.sub.x emissions usually have increased particulate emissions. On the contrary, methods for reducing particulates generally increase NO.sub.x emissions.
Surveys of the art appear to reflect that most of the work has been concentrated on the reduction of particulate emissions. This avenue of attack includes filter traps, catalytic and fuel additive systems designed to reduce fast ignition temperature and regeneration. On the other hand, work on NO.sub.x reduction from diesel engines has been somewhat limited. It is surmised that this difficult problem has been exacerbated by high oxygen levels in the exhaust stream. With the introduction of new and tighter standards scheduled to become effective in the near future, NO.sub.x reduction in diesel exhausts will require new catalytic systems capable of functioning effectively in high oxygen atmospheres.
Though a number of stationary systems have been successfully commissioned, no similar systems appear to exist in the transportation sector. The catalytic reduction of NO.sub.x by NH.sub.3 (ammonia) over a copper catalyst has been effectively demonstrated where fossil fuels are used to heat boilers, reactors, etc. It is not applicable to vehicles mainly because of the potential hazard posed by the on board storage of NH.sub.3, and to a lesser extent, because of the effects of low NH.sub.3 vapor pressures on the highly controlled delivery system required to match the rapid variations in NO.sub.x content during operation. In the laboratory, tests suggested that Cu and NH.sub.3 incorporated into zeolite resins may be effective in the catalytic reduction of NO.sub.x. However, its effectiveness as a deNO.sub.x catalyst seemed limited by its durability. Though the influence of diesel fuel-water emulsions (water/fuel ratios.ltoreq.0.25) on particulates and NO.sub.x is very significant, large increases of carbon monoxide (CO) and condensibles (polynuclear aromatic hydrocarbons [PAH]) make the process unattractive. Relatively small amounts of water in the emulsions give very large reductions in NO.sub.x and particulates.
A recent publication claimed to have electrocatalytically reduced NO.sub.x emissions from vehicles fueled by natural gas. See Marshalla M. Wright, et al, "Solid State Electrochemical Cell for Nitrogen Oxide (NO.sub.x) Reduction" Proc. Intersoc. Energy Convers. Eng. Conf. 27, Volumes 4, pages 4.321-4.325 (1992). The cathodes describe a honeycomb shaped ceramic coated with silver (or silver containing conducting material) in two distinct regions so that a cathode and anode were formed with an applied voltage across the system. Electrocatalytic reduction at the cathode resulted in the NO.sub.x going to N.sub.2 and 2O, as the exhaust gas flowed through the channels. The N.sub.2 continued through the cell while the O ions, dissolved in the solid electrolyte, usually consisting of zirconia (ZrO.sub.2), hafnia (HfO), titania (TiO.sub.2), or the lanthanide oxides, were converted to O.sub.2 at the anode. Although promising, this is very new technology and must yet clear several obstacles before it can be considered viable.
U.S. Pat. No. 5,106,802 discloses a diesel engine catalyst employing a honeycomb structure. A number of catalytic materials including silver are enumerated.